Interrupt return restores the frame created when ivss delivers an interrupt. The assembler expands iret into four machine loads. The first restores the saved helper register thirteen:

Elf_PushByte(elf, PACK(OC_LOAD, MOD_LOAD_3));
Elf_PushByte(elf, PACK(IDX, SP));
Elf_PushByte(elf, 0x00);
Elf_PushByte(elf, 0x04);

That encoding loads from the current stack pointer and increments it by four. The remaining generated instructions restore register twelve, %status, and %pc in that order.

At handler entry, the automatic frame has this layout:

sp +  0: saved r13
sp +  4: saved r12
sp +  8: saved status
sp + 12: saved next pc

The complete expansion consumes sixteen bytes. Any registers pushed explicitly by the handler must be popped first, so the stack pointer again identifies this frame.

ret restores only a return address and therefore cannot replace iret. The actual ivss expansion is several instructions rather than one atomic machine operation; its final load transfers execution back to the saved program counter.

Source: asembler.c.